Sensory and Motor Conduction Studies

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carpal tunnel syndrome

The electrodiagnostic techniques described in these books are of established values in the diagnosis of the median nerve entrapment neuropathies as the carpal tunnel syndrome (CTS) or anterior interosseous nervesyndrome (AIN), and they can be a valuable aid in the electrophysiological assessment of the polyneuropathies. The CTS is the most common entrapment neuropathy caused by the compression of the median nerve as it passes through the carpal tunnel at the wrist (1) and, consequently, it is one of the most frequent reasons for referral to electromyography laboratories for an electrodiagnostic (EDX) study (2). It is a clinical syndrome of numbness which usually involves the lateral four digits (or sensory loss may be confined to one digit), tingling, burning, and/or pain (3), and the hand pain may be perceived more proximally in the forearm (at the arm or at the shoulder level), mimicking cervical radiculopathy. Pain and paresthesia are aggravated by the repetitive use of the hand, and symptoms usually involve the dominant hand especially in persons who use their hands occupationally. Patients characteristically awaken at night with nocturnal paresthesia, and the onset is insidious in most cases. Weakness and atrophy of the thenar muscles is commonly observed in severe cases.

Recommendations and practice guidelines

The diagnosis is usually based on typical symptoms and signs in conjunction with electrodiagnostic findings (4). Many electrodiagnostic techniques have been developed to assist the electromyographer in the diagnosis of carpal tunnel syndrome (5), in order to verify the compression of the median nerve at the wrist and to assess its severity (6, 7).

In 1993, the American Academy of Electrodiagnostic Medicine’s (AAEM) Quality Assurance Committee performed a critical analysis of the CTS electrodiagnostic literature and concluded that the median sensory nerve conduction studies are more sensitive than the median motor nerve conduction studies, the short segment (wrist-palm segment) median sensory or mixed nerve conduction studies are more sensitive than the long segment (wrist-digit) sensory or mixed nerve conduction studies (8). The AAEM formed a second CTS task force in 2000 to update the 1993 report and to provide a single reference for EDX studies in CTS (9). In patients suspected of positive CTS, the following standard electrodiagnostic studies (practice standards) are recommended to confirm a clinical diagnosis of CTS:

1.

The median sensory nerve conduction studies (orthodromic or antidromic studies) across the wrist with a conduction distance of 13 to 14 cm (one test between S4, S5, S26, S27 and S39), as follows:

2.

If the result is abnormal, comparison of the result of the median sensory nerve conduction studies to the result of a sensory nerve conduction study of one other adjacent nerve (radial or ulnar nerve) in the symptomatic limb.

3.

If the result is normal, one of the following additional studies is recommended:

Comparison of the median sensory or mixed nerve conduction across the wrist (palm-wrist segment) over a short (7-8 cm) conduction distance with the ulnar sensory or mixed nerve conduction across the wrist over the same short (7-8 cm) conduction distance (S18 or S24), as follows:

Comparison of the median sensory conduction across the wrist with the radial or ulnar sensory nerve conduction across the wrist in the symptomatic limb (one test between S19, S21, S30 and S33), as follows:

Comparison of the median sensory or mixed conduction through the carpal tunnel (palm-wrist segment) to the sensory or mixed nerve conduction studies of proximal (wrist-elbow segment) or distal (digit-palm segment) segments of the median nerve in the symptomatic limb (one test between S16, S23 and S48), as follows:

The practice standards are generally accepted principles for patient management that reflects a high degree of clinical certainty. AAEM also provides practice guidelines, which are recommendations for patient management that reflect moderate clinical certainty:

4.

Motor conduction study of the median nerve recording from the thenar muscle (abductor pollicis brevis – APB muscle) and one other nerve in the symptomatic limb to include measurement of distal latency as the motor conduction study of the ulnar nerve recording from the hypothenar muscle (abductor digiti minimi, ADM muscle), as follows:

6.

Needle electromyography of a sample of muscles innervated by the C5 to T1 spinal roots, including a thenar muscle (abductor pollicis brevis – APB muscle) innervated by the median nerve of the symptomatic limb.